1. Field of the Invention
The present invention relates to variable reluctance dynamic force motors (or actuators) used to convert electrical energy into oscillating or reciprocating mechanical force.
2. Description of the Related Art
To produce a high reciprocating force with reasonable transduction efficiency, one often employs a variable reluctance electromagnetic motor (VR motor). In particular, VR motors are generally lighter and more efficient than electrodynamic (voice coil) motors capable of producing similar peak forces. However, VR motors, as commonly controlled, can exhibit substantial instabilities that must be overcome before useful operation is attained. These instabilities can be particularly severe when the motor stroke is a high percentage of the nominal gap length, a situation often encountered when driving dynamically reactive loads. Also, transient disturbances can also lead to instabilities in VR motors controlled according to common prior art methods.
Representative examples of the manner in which VR motors have traditionally been analyzed and operated can be found in U.S. Pat. Nos. 5,206,839 and 5,266,854 to Murray, and U.S. Pat. No. 5,375,101 to Wolfe et al. The method of control described in this prior art can be generally described as xe2x80x9ccurrent control.xe2x80x9d The following analyses (and assumptions) are common to these patents:
1. The flux established in a gapped electromagnetic device is proportional to the current in the electromagnet coil. Since the actual current-to-flux relation is described by xcfx86xcx9ci/g (where xcfx86 is the flux in the gap, i is the current in the coil, and g is the length of the air gap), this assumption is only valid when the gap length g is constant.
2. The force developed across the air gap of a gapped electromagnetic device is proportional to the square of the established flux (Fxcx9cxcfx862.
3. Therefore, Fxcx9ci2.
If VR motors could be operated effectively by simply appealing to point 3 (namely, that Fxcx9ci2), then one could generate an arbitrary force response by simply controlling the motor coil current to be the square root of the desired force. Unfortunately, for a dynamic force motor to deliver any real power to a load, the xe2x80x9cmovablexe2x80x9d part must, in fact, experience displacement with respect to the xe2x80x9cstationaryxe2x80x9d part (the xe2x80x9cmovablexe2x80x9d and xe2x80x9cstationaryxe2x80x9d parts of the motor are henceforth referred to as the armature and core, respectively). In other words, the gap length g in point 1 is not constant. Operation in this manner (allowing the motor parts to move) violates the xe2x80x9cno gap motionxe2x80x9d assumption expressed earlier, and, to a greater or lesser extent, invalidates the Fxcx9ci2 relationship. Or, put in another way, the actual relationship to consider is Fxcx9c(i/g)2.
One way of overcoming this complication of the Fxcx9c(i/g)2 relation is to require that the motor air gap(s) remain nearly constant. For example, if a motor has a nominal 1 mm unenergized air gap, one might choose to limit relative dynamic motion to 0.05 mm, or a 5% excursion of the nominal gap length. It is generally accepted by those skilled in the art that if the gap motion is tightly constrained in this manner, then Fxcx9ci2 reasonably represents the behavior of the motor. Furthermore, schemes that establish a substantial bias current in the motor coils and then employ small-signal perturbation techniques with respect to this bias point are common in the prior art. Prior art vibrators, such as the one described in U.S. Pat. No. 3,775,626 to Brosch, have noted that the air gap length must, for some material handling applications, be set large enough to be robust to changes in the operating conditions of the vibratory material handling system.
However, those skilled in the art will also appreciate that, for a given desired dynamic force, one can minimize the corresponding motor coil currents by minimizing the length of the air gap between the armature and the core. To do this, and still have a motor that can experience relatively large strokes, one must allow large gap excursions (up to 100% of the nominal unenergized gap length). In the presence of large gap excursions, the Fxcx9ci2 relation is no longer adequate to describe the behavior of the motor, and one must deal with the added nonlinearity exhibited by Fxcx9c(i/g)2.
An important attribute of the present invention (to be discussed in detail in another section) is the inherent stability that it provides in the context of VR motor control. In classical control theory, stability means that the output of the VR motor (either the relative positions of the armature and core, or the force generated between them) will not grow without bound due to a bounded input, initial condition, or unwanted disturbance. In other words, there will always be a reasonable, bounded relationship between the motor action requested via an input signal and the resulting actual motor action, even in the presence of external noise, transients, or other disturbances.
When evaluating the control stability consequences of the Fxcx9ci/g)2 relationship that has been discussed previously, it is instructive to consider a simple example. In a one-sided motor structure such as that shown in FIG. 1, start with an initial fixed coil current lo that flows through coil 4, and an initial gap length 12 G0 (denoted by label 12 in FIG. 1). Thus, the initial force between the two moving motor parts (core 2 and armature 8) will be, by Fxcx9c(ig)2, proportional to (I0/G0)2. As this initial force acts to move the two motor parts closer together, the instantaneous gap length will decrease to values smaller than G0, and as the gap reduces, the instantaneous force will increase as 1/ g2, even while the coil current is held constant. With most common VR motors, the suspension stiffness is linear (F=xe2x88x92kx), but this increase in electromagnetic force is quadratic (1g2), so the net effect can be a rapidly increasing force that moves the motor parts together until they collide and clamp. Or, for motor designs such as those described in U.S. Pat. No. 5,266,854 to Murray, the suspension can be stiff enough so that for small displacements from the nominal gap, the motor is stable, but for larger displacements, one experiences such xe2x80x9crunawayxe2x80x9d instability. In particular, see column 3 and FIG. 3 of U.S. Pat. No. 5,266,854 to Murray for further details. To more fully stabilize such xe2x80x9ccurrent-controlledxe2x80x9d motors, one must vary the coil current in relation to the instantaneous gap length or measured magnetic flux via a feedback mechanism, and even these feedback circuits can suffer from bandwidth and gap displacement limitations.
Examples of prior art that addresses these concerns may be found in U.S. Pat. No. 5,621,293 to Gennesseaux, and xe2x80x9cParametric Modeling and Control of a Long-Range Actuator Using Magnetic Servo Levitationxe2x80x9d, IEEE Transactions on Magnetics, authored by H. Gutierrez and P. Ro (September, 1998). In Gennesseaux, gap-motion-induced instabilities are compensated via two distinct means: 1) measuring the actual armature motion with a displacement sensor and compensating the coil current accordingly and 2) measuring the air gap flux with a Hall-effect flux sensor and compensating the coil current accordingly. In Gutierrez and Ro, the authors attempt to compensate not only the gap-motion nonlinearities, but also distortions to the first-order relations introduced by material non-idealities, flux density non-uniformity, and the like. In both of these cases, extensive and complex controllers are used to achieve gap excursions larger than is common in the traditional art, but in neither case is evidence presented that large gap excursions, such as gap excursions approaching 100% are possible. U.S. Pat. No. 3,219,919 to Snavely also uses a position transducer and feedback circuit to compensate the Fxcx9c(i/g)2 relation.
It is worth noting that, historically, there have been a few predominant applications for VR motors. The first is for acoustic transduction, particularly in the generation of high-amplitude sound waves for underwater applications (sonar systems, for example). In such acoustic applications, designers were primarily interested in flat wide-band frequency response, stable transducer operation, and low distortion output. Similarly, VR motors are sometimes used to excite mechanical structures for the purpose of performing modal analyses, and for these applications, similar motor characteristics are valued. Generally, though, the control schemes common in the prior art are not sufficiently robust for the high gap excursions and highly dynamic reaction forces exhibited by such loads as resonant acoustic compressors.
Another predominant application for motor structures of this sort is for vibrator applications, especially in the context of vibratory material handling (conveying sand, beans, rocks, and other granular materials). Primarily, the vibrator community is interested in physically robust vibratory devices, simple operation, and low cost. Typical vibrators are run at 50/60 Hz (line frequencies) or integer multiples or sub-multiples of these frequencies.
As discussed above, the VR motor/vibrator communities have often resorted to closed-loop feedback schemes to enhance the stability of VR motors. Ironically, the present invention demonstrates that, properly understood, maximum stability when operating a dynamic force VR motor is achieved by simply removing all feedback mechanisms, leading one to the unobvious result that the highest operational stability with such devices is achieved with no feedback at all.
An object of this invention to overcome the aforementioned limitations formerly associated with VR motors by providing an apparatus and/or method whereby a VR dynamic force motor can be operated stably over a wide range of frequencies with gap excursions approaching 100%.
Another object of this invention is to provide a VR motor drive scheme that demonstrates low-distortion characteristics previously associated with current-control schemes via open-loop, stable, voltage control schemes.
Another object of this invention is to provide for large gap excursion operation with relatively simple, high efficiency control schemes.
Another object of this invention is to provide for variable amplitude operation of a VR motor where the force amplitude is controlled via relatively simple, open-loop control schemes.
Another object of this invention is to provide for large gap excursion operation with efficient, open-loop drive systems that employ no feedback.
Another object of this invention is to provide xe2x80x9cflatxe2x80x9d force output over a wide range of frequencies while not resorting to the current feedback approaches common in the prior art.
Another object of this invention is to provide open-loop-stable operation of a VR motor when driving dynamically reactive loads such as acoustic compressors or other mechanically resonant systems.
Another object of this invention is to provide open-loop-stable, large gap excursion operation of VR motors that simultaneously generate dynamic force at multiple operating frequencies.
Another object of this invention is to provide for operation of VR motors that in some cases approaches the ideal case without employing any flux feedback.
At least one of the above-mentioned objects and advantages may be achieved by a method of controlling a VR motor, which includes providing, via an open-loop active circuit, a dynamic voltage to a coil of said motor to result in a substantially unipolar current flowing in the coil, said periodic voltage being applied so as to maintain control of said motor irrespective of instantaneous motor gap and driven load characteristics.
At least one of the above-mentioned objects and advantages may also be achieved by a method of controlling a VR motor that is operable at a plurality of operating frequencies. The method includes receiving a signal that includes a plurality of discrete frequencies, as a command input to the VR motor. The method also includes precompensating the command input in accordance with a 1/xcfx89 characteristic of the VR motor. The method further includes generating a low-distortion force representative of the command input.
At least one of the above-mentioned objects and advantages may also be achieved by a method of controlling a VR motor, which includes providing, via an open-loop active circuit, a bipolar periodic voltage to a coil of said motor to result in a substantially unipolar current flowing in the coil, said periodic voltage being applied so as to maintain control of said motor irrespective of instantaneous motor gap and driven load characteristics, said bi-polar periodic voltage being derived from a uni-polar voltage source.
At least one of the above-mentioned objects and advantages may also be achieved by a method of controlling a VR motor, which includes providing a dynamic voltage to a coil of said motor so that a substantially unipolar coil current flows in the coil, said periodic voltage being applied so as to maintain control of said motor irrespective of instantaneous motor gap and driven load characteristics. The method also includes measuring the current in a conductive winding of said VR motor. The method further includes generating a DC offset voltage that is summed with said applied dynamic voltage such that a time duration and magnitude of excursions of said measured current are minimized.
At least one of the above-mentioned objects and advantages may also be achieved by a method of generating a substantially sinusoidal flux waveform for a variable reluctance (VR) motor, which includes applying a voltage waveform to a coil of the motor, the voltage waveform having a substantially zero mean, the voltage waveform being maintained irrespective as to a change in a size of the gap, wherein no current control with respect to the coil is utilized during operation of the motor.
At least one of the above-mentioned objects and advantages may also be achieved by a method of controlling a variable reluctance (VR) motor, which includes applying a substantially zero-mean voltage waveform to a coil of the motor. Each cycle of the substantially zero-mean voltage waveform includes: 1) a first time period in which a first positive voltage value is provided, 2) a second time period in which a zero voltage value is applied, and 3) a third time period in which a second negative voltage value is applied, wherein an absolute value of the first and second voltage values is substantially equal to each other.
At least one of the above-mentioned objects and advantages may also be achieved by a method of controlling a multi-frequency variable reluctance motor that operates in at least a first operation frequency and a second operating frequency, which includes calculating a sinusoidal voltage to be applied to a coil of the motor, the sinusoidal voltage being calculated as A sin(xcfx891)+B sin(xcfx892)t+C, wherein A, B and C are constants. The method also includes applying the sinusoidal voltage to the coil for at least a plurality of cycles of the sinusoidal voltage, the sinusoidal voltage being applied irrespective as to a gap width change between the core and the armature.
At least one of the above-mentioned objects and advantages may also be achieved by a variable reluctance (VR) motor, which includes a core having a coil. The motor also includes a movable part that moves based on a force provided in a gap disposed between the core and the armature. The motor further includes a voltage source that is configured to apply a dynamic voltage waveform to the coil that results in a substantially unipolar current flowing in the coil, the dynamic voltage being applied so as to maintain control of said motor irrespective of instantaneous motor gap and driven load characteristics.
At least one of the above-mentioned objects and advantages may also be achieved by a control apparatus for a two-sided variable reluctance motor that has an armature and a core with a first coil on the core and a second coil on the core, and with a first air gap between the first coil and the armature and a second air gap between the second coil and the armature. The control apparatus includes a voltage source that provides a voltage waveform to the first and second coils. The control apparatus also includes a first diode that is provided between the voltage source and the first coil. The control apparatus further includes a second diode that is provided between the voltage source and the second coil, wherein an anode of the first diode and a cathode of the second diode are directly connected to the first and second coils, respectively.
At least one of the above-mentioned objects and advantages may also be achieved by a control apparatus for a variable reluctance motor that has a core with a coil and an armature. The control apparatus includes an offset voltage source for providing a DC offset voltage. The control apparatus also includes a voltage source for outputting a sinusoidal voltage to the coil that is offset by the DC offset voltage. The control apparatus further includes a current sensing unit for sensing current presently passing through the coil. The control apparatus still further includes a rectifying circuit that removes all portions of the sensed current except for negative current pulses. The control apparatus also includes a low-pass filter that filters and output of the rectifying circuit, wherein an output of the low-pass filter is provided to the offset voltage source to provide an adjustment signal to either increase, decrease or maintain a current value of the DC offset voltage.
An understanding of the operation of the invention can be gained by utilizing the following relations that describe the behavior of VR motors:
1. To a first approximation, the force generated across the air gap of a VR motor is proportional to the square of the magnetic flux established in that gap (Fxcx9cxcfx892). This relation assumes that the flux density in the air gap of the motor is uniformly distributed, an assumption that is commonly regarded as reasonable for small gap VR motors by those skilled in the art.
2. For gapped magnetic circuits such as those discussed herein, the relationship between the voltage applied to the motor coil and the resulting magnetic flux is v=N(dxcfx89/dt)+iR, where v is the voltage applied to the coil, N is the number of turns of wire that constitute the coil, xcfx89 is the magnetic flux in the air gap, t is time, i is the current in the coil, and R is the resistance of the coil. This relation is commonly known as Faraday""s Law.
3. For gapped magnetic circuits such as those discussed herein, the relationship between the coil current and the magnetic flux in the air gap is xcfx89xcx9ci/g.
Using these relations, the following features have been found:
1. Except for the iR term in the v=N(dxcfx89/dt)+iR relation, the relationship between coil voltage and magnetic flux is independent of gap. The present invention has been developed based in part on the finding that the presence of this iR term does not adversely affect the operation of the invention, especially when not operating at low frequencies.
2. The magnetic flux is guaranteed to be zero whenever the coil current is zero, regardless of the instantaneous gap length.
In the present invention, stable, high gap excursion operation of VR dynamic force motors is accomplished by combining voltage control (applying an open-loop voltage v to a motor coil yields flux       φ    =                  ∫                                            v              -                              i                ⁢                                  xe2x80x83                                ⁢                R                                      N                    ⁢                      ⅆ            t                              +      C        )
and by employing ways to ensure that the current periodically returns to zero (which guarantees that the constant C in   φ  =            ∫                                    v            -                          i              ⁢                              xe2x80x83                            ⁢              R                                N                ⁢                  ⅆ          t                      +    C  
remains at a value between zero and the peak flux).